Touching the Unseeable

By N. A. Burnham

For centuries, scientists have been fascinated by small objects, starting with Robert Hooke's work that led to the publication of Micrographia in 1665. The optical microscopes of Hooke's era allowed a resolution roughly one-tenth the diameter of a human hair. In the 1930s, the invention of the electron microscope, in which samples are "illuminated" with high-energy electrons, rather than photons, allowed scientists to see beyond the wavelength of light.

Although the best electron microscopes can image atoms, there are limitations. Only conducting samples that reflect electrons or samples that can be coated with a conducting material can be viewed. Preparing samples for the highest-resolution imaging damages them. And the data reflect how samples absorb and emit electrons from a volume near the surface, rather than revealing the properties of the surface alone.

In 1981, two researchers at IBM in Switzerland invented a revolutionary device, called a scanning tunneling microscope (STM), in which a low-energy current of electrons passes between the sample and a very sharp needle. (The technology won them the Nobel Prize in physics just five years later.) The needle is moved back and forth with atomic precision, while the value of the current, which is exponentially related to the distance between the needle and the sample, is recorded and presented as a three-dimensional image. No destructive sample preparation is necessary, and the low energy of the electrons allows scientists to study just the sample's surface.

Atomic resolution is typical for STMs, and it stems from the nature of the interaction between the tip of the needle and the sample. They don't touch, but are held close enough to each other that the electrons "tunnel" (a quantum-mechanical effect) through the space between the tip and sample.

Above, from left, postdoctoral student Deli Liu, Burnham, and Keeley Stevens '07 prepare the AFM to probe a sample. The triangular arm that holds the cantilever can be seen on the center monitor, illuminated by laser light.

At WPI, the AFM has been used to investigate the mechanical properties of everything from carbon nanotubes and metal nanowires to the substrates upon which cells are cultured and studied.

The STM sired a family of about 100 instruments, now known collectively as scanning probe microscopes. The proliferation began in 1986 with the invention of the atomic-force microscope (AFM), now the most popular member of the family. Conceptually, the AFM is identical to the STM, except that it uses a fine needle to measure force instead of tunneling current.

The AFM's needle is manufactured on a flexible arm that moves in response to forces acting on the needle's tip. The arm's motion is detected, usually with a laser and photodiode, and the movements are translated into images. The integrated cantilever and needle are made using techniques developed for computer chip fabrication, allowing the radii of the needle tips to be only a few tens of nanometers—a thousand times smaller than a phonograph needle.

An AFM image showing triangles of aluminum on the polycarbonate substrate of a CD-Rom; the black dots are the data bits. The lighter the color, the higher the elevation; the range is about 400 nanometers, the wavelength of violet light.
Click here for a larger version.

The AFM can be used with an enormous range of samples (from computer chips to DNA), and its reasonable cost and simple design have made it popular among educators and researchers. The AFM offers another advantage: it can be used as a very fine finger to probe the mechanical properties of samples at a submicron (or even atomic) scale. This is my own expertise.

At WPI, the AFM has been used to investigate the mechanical properties of everything from carbon nanotubes and metal nanowires (some as stiff as a diamond) to the substrates upon which cells are cultured and studied (most as compliant as gelatin). Our "nanofingers" are also being used to probe the adhesion between microsensor surfaces and between bacteria and medical implants, and to observe crystal growth.

The AFM's usefulness has brought together faculty from several WPI departments, including Biomedical, Chemical, and Mechanical Engineering, and several neighboring institutions, such as the University of Massachusetts Medical School. My own interest in nanoscale metrology has led to work with the National Institute of Standards and Technology, and my interest in adhesion between microsensor surfaces has brought in industrial support from Analog Devices, a manufacturer of accelerometers for automotive safety and videogame consoles.

Soon after I arrived at WPI in January of 2000, professor Thomas Keil, then head of the Physics Department, suggested that I offer a course in nanoscience to undergraduates. I decided to restrict the course content to the AFM, reasoning that students who did well in the class and lab would make good project students. Thus, Atomic Force Microscopy at WPI was born. Ninety students, from freshmen to a professor emeritus, from physicists to biologists, have enrolled or audited the course since 2001. My experience as an AFM educator is slowly turning into a textbook for undergraduates, supported in part by the National Science Foundation.

It is easy to attract students to the course. They are excited by the prospect of visualizing and probing structures smaller than the wavelength of light, and about getting to use what has become the key enabling tool for research in nanoscience and the development of nanotechnology. Besides, who can resist the allure of touching the unseeable?